1. Field of the Invention
This invention relates to an improved method and apparatus for the gasification of cellulosic biomass to produce low BTU gas to be used as a fuel in various agricultural and industrial applications.
2. Description of the Prior Art
The consumption of combustible solid to produce low BTU gas products in a most efficient manner has received much attention in the past several years due to the energy shortage. The search for other sources of fuel for agricultural and industrial applications has led to the construction of gasifiers which use organic material in the form of refuse, wood or other solid carbonaceous material.
The burning of refuse material in incinerators which use excess air to limit flame temperatures have produced large amount of effluent gases to be handled by gas cleaning equipment which must be of a tremendous size in order to handle the volume of gas generated. Other problems with regard to normal incinerator type of operation involves the filtration of incinerator fumes which have proved impractical because the odors generated are of a complex chemical nature not possible to filter out. Other absorption and catalytic agents including masking agents used as counter-odorents have proved equally unsatisfactory. Industrial gas producers of the prior art type use separation, drying and grinding and other preparation prior to the actual gasification process with a downward gravitation flow of the refuse fuel and an upward flow of the gaseous and vaporous products. This upward flow of the vaporous products provides difficulty in the collection process and tends to harm the structure of the furnace due to the nature of the smoke products formed on the surfaces. Furthermore, these gasification apparatus' operate at pressures exceeding ambient atmospheric pressure such that leakage of noxious fumes can be a problem.
One of the most common techniques of providing a conversion of biomass material to gas involves the use of a dumping of the material into container which has a grate near the bottom. The material is mixed with combustible air which is force-fed and the burning product produces the gas which is pumped away from the source. The purpose of the grate is to provide a surface which allows for a complete burn up of the product and the removal of the ash away from the combustion area so that the heat is not "washed" on completely combusted material.
Other construction such as shown by the patent to Giddings U.S. Pat. No. 3,746,521 disclose the removal of the waste material by means of a ram 41 which essentially functions to push out the material at the bottom of the combustion chamber. This construction of a ram, while providing for the removal of material does not function in an efficient manner with regard to the burning of the product because the ram pushes away material which may contain some product not fully burned and thus reduces the efficiency of the process. Thus the use of a grate in this type of biomass gasification process is seen to be the most reliable for efficiency of conversion and minimization of heat loss on already fully combusted products.
The prior art type of constructions involving the use of grates have mainly circular type grates which are placed at the bottom or near the bottom of a gasification chamber. These grates essentially must not only function to filter the completely combusted product but also must serve to aid or, in some cases fully support the suspension of the fuel material. That is these grates must be strongly constructed so that they can withstand the weight of the biomass which is being converted to gas. This can many times be a problem because of the nature of the material of the grate as well as the high temperatures involved in the process which could weaken the biomass suspension ability of the grate. This is particularly true in large volume gasifiers where the weight of the biomass becomes a problem on the grate structure. This problem is particularly relevant when the grate is of a circular construction due to the nature of the high temperatures which occur at the support device for the grates and at the surface of the grate.
Other problems involved in the prior art use of grate construction include the necessity for increasing the area of a circular grate each time the biomass handling capacity was to be increased which further increased the problem with regard to materials. Likewise the large circular zone designs for grates and therefore for combustion zones provides problems with scaling production capacities and requires the use of complex gas removal and ash removal mechanisms.
The present invention proposes a biomass gasifier which uses a linear construction of the grate and combustion zones in combination with a down-draft operation to provide a gasifier which overcomes the above-mentioned problems.